✨ Add Items (crud, models, endpoints), utils, refactor (#14)

* Update CRUD utils to use types better.
* Simplify Pydantic model names, from `UserInCreate` to `UserCreate`, etc.
* Upgrade packages.
* Add new generic "Items" models, crud utils, endpoints, and tests. To facilitate re-using them to create new functionality. As they are simple and generic (not like Users), it's easier to copy-paste and adapt them to each use case.
* Update endpoints/*path operations* to simplify code and use new utilities, prefix and tags in `include_router`.
* Update testing utils.
* Update linting rules, relax vulture to reduce false positives.
* Update migrations to include new Items.
* Update project README.md with tips about how to start with backend.

{{cookiecutter.project_slug}}/backend/app/app/db/init_db.py

@@ -1,6 +1,6 @@
 from app import crud
 from app.core import config
-from app.models.user import UserInCreate
+from app.models.user import UserCreate
 
 
 def init_db(db_session):
@@ -11,7 +11,7 @@ def init_db(db_session):
 
     user = crud.user.get_by_email(db_session, email=config.FIRST_SUPERUSER)
     if not user:
-        user_in = UserInCreate(
+        user_in = UserCreate(
             email=config.FIRST_SUPERUSER,
             password=config.FIRST_SUPERUSER_PASSWORD,
             is_superuser=True,